The invention relates to alkali-free aluminoborosilicate glasses. The invention also relates to uses of these glasses.
High requirements are made of glasses for applications as substrates in flat-panel liquid-crystal display technology, for example in TN (twisted nematic)/STN (supertwisted nematic) displays, active matrix liquid crystal displays (AMLCDs), thin film transistors (TFTs) or plasma addressed liquid crystals (PALCs). Besides high thermal shock resistance and good resistance to the aggressive chemicals employed in the process for the production of flat-panel screens, the glasses should have high transparency over a broad spectral range (VIS, UV) and, in order to save weight, a relatively low density xcfx81, preferably xe2x89xa62.600 mg/cm3. Use as substrate material for integrated semiconductor circuits, for example TFT displays (xe2x80x9cchip on glassxe2x80x9d) in addition requires thermal matching to the thin-film material a-Si or polysilicon (xcex120/300≈3.7xc2x710xe2x88x926/K) and the absence of alkali metal ions. Sodium oxide contents of less than 1000 ppm as a result of production can be tolerated with respect to the generally xe2x80x9cpoisoningxe2x80x9d action due to diffusion of Na+ into the semiconductor layer.
Suitable glasses should be capable of being produced economically on a large scale in adequate quality, for example in a float plant or by the drawing method. In particular, the production of thin ( less than 1 mm) streak-free substrates with low surface undulation by the drawing process requires high devitrification stability of the glasses. Compaction of the substrate during production, which has a disadvantageous effect on the semiconductor microstructure, can be countered by establishing a suitable temperature-dependent viscosity characteristic line of the glass: with respect to thermal process and shape stability, it should have a sufficiently high glass transition temperature Tg, i.e. a glass transition temperature Tg of at least 650xc2x0 C., while on the other hand not having excessively high melting and working (VA) points, i.e. a VA of xe2x89xa61330xc2x0 C.
The demands made of glass substrates for LCD display technology are also described in xe2x80x9cGlass substrates for AMLCD applications: properties and implicationsxe2x80x9d by J. C. Lapp, SPIE Proceedings, Vol. 3014, invited paper (1997).
Corresponding requirements in principle are made of glasses for substrates in thin-film photovoltaics, especially based on microcrystalline silicon (xcexcc-Si).
An essential prerequisite for the commercial success of thin-film photovoltaics over high-cost solar technology based on crystalline Si wafers is the presence of inexpensive high-temperature-resistant substrates.
At present, two different coating processes are known for the production of thin-film solar cells based on crystalline Si. A process which has proven particularly favourable with respect to high layer quality and thus efficiency and high deposition rates is a high-temperature CVD process using inexpensive trichlorosilane as Si source. This process proposes the direct deposition of thin crystalline Si layers and requires the heating of a suitable substrate to 1000xc2x0 C. or above. The only suitable substrates are then comparatively expensive ceramics, graphite, silicon or similar materials. The use of glass-ceramics, which are likewise expensive, is also being discussed.
As an alternative to this, low-temperature Si deposition processes are being developed which allow the use of the less expensive substrate material glass. One possibility here is the deposition of amorphous silicon at low temperatures of up to 300xc2x0 C. and, in a subsequent step, the recrystallization of the layers, for example using laser or zone-melting methods, with formation of partly crystalline xcexcc-Si layers. In order to prevent deformation of the glass plate at the temperatures prevailing in the heating process, a special glass with very high heat resistance which is matched thermally to silicon is necessary, as is the case in glasses having glass transition temperatures Tg of at least 650xc2x0 C. As a consequence of the tendency to change over from a-Si to crystalline poly-Si coatings, the highest possible heat resistance of the substrate is also desired for substrates for TFT display technology. Since the thermal expansion of the Si layers decreases with increasing crystallinity thereof, glasses having very low expansion xcex120/300 of up to 3.0xc2x710xe2x88x926/K or less are also desired here.
The current development of xcexcc-Si technology is moving in the direction of a substrate concept, i.e. the support material forms the basis of the solar cells and the incident light does not pass through it. In addition, a development towards a less expensive superstrate arrangement (light passes through the substrate material, no cover glass necessary) is not excluded. In order to achieve high efficiencies, high transparency of the glass in the VIS/UV is then necessary, which means that the use of semi-transparent glass-ceramics, besides the abovementioned cost reasons, proves to be disadvantageous.
For the last-mentioned application (superstrate) and for said applications as display substrates, the quality of the glasses with respect to the number and size of flaws, such as solid inclusions and bubbles, is of great importance since they impair the transparency of the glasses.
The glasses should thus if possible be free from or at least have a low content of bubbles and streaks. In particular of the glasses processed by the float method, a well fined glass can only be obtained with difficulty since the known effective fining agents As2O3 and Sb2O3 cannot be employed owing to their ease of reduction under the conditions of the float bath.
Similar demands are also made of glasses for light bulbs: glasses for halogen lamps must be essentially free from alkali metals since alkali metal ions disrupt the regenerative halogen cycle of the lamp. The glasses must have high thermal stability since high bulb temperatures usually occur in operation. The glasses must be sufficiently stable to devitrification in order to be suitable for tube drawing. For use as lamp envelope glass for light bulbs which contain molybdenum components as electrode or lead material, the thermal expansion of the glasses must be matched to that of molybdenum (xcex120/300=5.0xc2x710xe2x88x926/K) in order that leak-tight, stress-free fusing is achieved between the metal and the glass. Also for this use, the glasses should have the lowest possible bubble content.
The complex property profile outlined above is achieved best by borosilicate glasses from the alkaline earth metal aluminoborosilicate glass sub-family. Commercially available glasses for TFT and AMLCD applications also belong to this glass type. Numerous patent specifications which describe glasses for said uses are also already known:
EP 672 629 A2 and U.S. Pat. No. 5,508,237 describe aluminosilicate glasses for flat-panel displays. They exhibit various composition ranges with various coefficients of thermal expansion. These glasses can allegedly be processed not only by the overflow fusion drawing process, but also by other flat-glass production methods. However, the glasses, in particular, which have a coefficient of thermal expansion matched to polycrystalline Si will have very high working points VA, which make them unsuitable for the float process. The visual quality of the glasses will not be high since no method for effective, in particular float-compatible fining is indicated. The fining agents Sb2O3 and As2O3 mentioned by way of example are unsuitable for the float process owing to their ease of reduction. The same applies to the optional glass components Ta2O5 and Nb2O5. The same applies to the alkali-free glasses for TFT display applications which are described in the specifications U.S. Pat. No. 5,374,595, WO 98/270 19, EP 714 862 A1, EP 341 313 B1 and JP 10-722 37 A. The glasses of JP 8-295530 A, JP-9-48632 A and JP 9-156 953 A likewise cannot be fined effectively.
A similar, albeit lesser deficit is also exhibited by the glasses having broad composition ranges from the specifications WO 97/11919 and WO 97/11920, even though SnO2 is mentioned here at least as an optional fining agent.
JP 10-139467 A describes alkali-free glasses having high acid resistances for display applications in which SnO2, TiO2 and ZrO2 are in each case optional components, their sum being at least 0.1 mol %. No further fining details are given.
The good fining action of SnO2 in alkali-free alkaline earth metal aluminoborosilicate glass melts is documented in the patent specifications DE 196 17 344 C1 and DE 196 03 698 C1. The tin-containing glasses therein not only have good basic glass properties for their use as TFT display glass, but can also be produced with high product quality. The unexamined Japanese publication JP 10-59741 A also describes SnO2-fined alkali-free glasses, again from a broad composition range.
In the case of the zinc-free aluminoborosilicate glasses described in the patent specification DE 197 39 912 C1, it has been found that the combination of SnO2 and CeO2 significantly exceeds the fining action of SnO2 through stabilization of the SnO2/SnO redox equilibrium. The very crystallization-stable glasses described therein can be produced and processed on a large industrial scale both by the float process and by a wide variety of drawing processes, such as microsheet down-draw or overflow fusion processes.
By contrast, JP 10-130034 A and JP 10-114538 A indicate the particular fining action of the As2O3+SnO2 and Sb2O3+SnO2 combination. In contrast to combined SnO2/CeO2 fining, however, there is a considerable potential risk regarding corrosion of platinum material parts in the interface and hot-shaping area. DE 196 01 922 A1 describes SnO- and ZrO2-containing alkali-free alkaline earth metal aluminoborosilicate glasses which have very good crystallization stabilities, although both SnO and ZrO2 are said to increase the tendency towards crystallization if they are added individually.
An object of the present invention is to provide glasses which have high internal glass quality, are stable at high temperatures, have good melting and processing properties, have good chemical resistance and have coefficients of thermal expansion xcex120/300 of from 2.8 to 5.0xc2x710xe2x88x926/K.
This object is achieved by the glasses described herein. Uses of the glasses are also described.
The glasses comprise 50-70% by weight of the network former SiO2. At low contents, the chemical resistance and the heat resistance of the glasses are impaired. Higher contents of SiO2 increase the viscosity of the melt at high temperatures and thus impair the melting properties of the glasses. Its devitrification tendency also increases, it being possible for, in particular, crystalline precipitations of SiO2 to occur as cristobalite. The content is preferably 53-68% by weight, particularly preferably  greater than 55% to 65% by weight, very particularly preferably  greater than 58% by weight of SiO2.
The Al2O3 content can be from 10 to 25% by weight. Higher contents have an adverse effect on the process temperatures during hot shaping, since the working point VA would be greatly increased. Lower contents increase the crystallization susceptibility of the glasses and lower the glass transition temperatures Tg and thus the thermal stability of the glasses. The content is preferably from 12 to 24% by weight, particularly preferably from  greater than 14% to 23% by weight.
B2O3, likewise a network former, is present in the glasses in an amount of 0.5-15% by weight. Higher contents would reduce the chemical resistance to acids and the thermal stability of the glasses. At lower contents, the melting-simplifying action of the B2O3 or of the boric acid would not yet be very pronounced, and the crystallization stability would be reduced. The contents are preferably from 3, especially from 4, to 13% by weight, particularly preferably from 5 to 11% by weight.
The total alkaline earth metal oxide content and its specific implementation by means of one or preferably more oxides can vary within a relatively broad range, giving thermal expansions of from xcex120/300=2.8xc2x710xe2x88x926/K to xcex120/300=5.0xc2x710xe2x88x926/K: thus, the glasses can contain 0-10% by weight of MgO, 0-12% by weight of CaO, 0-12% by weight of SrO and 0-15% by weight of BaO, where the sum of these oxides is at least 8% by weight and at most 26% by weight, preferably from 8 to 24% by weight, particularly preferably from 8 to 21% by weight. An alkaline earth metal oxide content of at least 9% by weight is very particularly preferred. In particular for glasses having rather low expansion coefficients (xcex120/300xe2x89xa64.0xc2x710xe2x88x926/K), it is preferred to restrict the sum of the alkaline earth metal oxides to a maximum of 18% by weight. With the aim of a low temperature at viscosity 104 dPas, good melting properties and low density, the presence of the lightweight oxides MgO and/or CaO is preferred. Both oxides are preferably present. Their maximum content is preferably 9% by weight each. A minimum content of MgO of at least 0.5% by weight is preferred. A content of 0.5-8% by weight of CaO and 3.3-8% by weight of MgO is particularly preferred. Higher contents than the 10 and 12% by weight mentioned would lower the chemical resistance, in particular to buffered hydrofluoric acid solution, and the devitrification stability. A minimum content of  greater than 4% by weight of MgO is very particularly preferred.
Although it is advantageous in respect of simple batch production to employ as few different alkaline earth metal oxides as possible, even the presence of small amounts of further oxides increases the crystallization stability of the glasses. SrO and BaO in particular have an advantageous effect on the heat and crystallization stability. The presence of SrO is therefore also preferred. A maximum content of 9% by weight of SrO and 12% by weight of BaO is preferred. A content of 0.5-8% by weight of SrO and the additional presence of BaO in a maximum amount of 10% by weight is particularly preferred.
Particularly high-temperature-resistant glasses have low B2O3 contents, preferably  less than 5% by weight, and relatively high SiO2 contents. Glasses having high coefficients of expansion (xcex120/300xe2x89xa74xc2x710xe2x88x926/K) have rather high contents of alkaline earth metal oxides, preferably up to 25% by weight. Thus, glasses which are particularly suitable as high-temperature-resistant glasses, for example as high-temperature-resistant lamp glasses, have low B2O3 contents, preferably even xe2x89xa63% by weight, and comparatively high SrO and in particular high BaO contents. They thus have comparatively high density. Glasses having low coefficients of expansion (xcex120/300xe2x89xa64.0xc2x710xe2x88x926/K) contain rather less alkaline earth metal oxides, preferably xe2x89xa618% by weight, preferably composed of up to 7% by weight of MgO, up to 7% by weight of CaO, up to 7% by weight of SrO and up to 5% by weight of BaO. Particular preference is given in these classes to network former contents of 60xc2x12% by weight of SiO2, 17xc2x12% of Al2O3 and 7.5xc2x11.5% of B2O3.
The glasses according to the invention may furthermore contain up to 10% by weight of ZnO, preferably up to 5% by weight of ZnO. With its effect on the viscosity characteristic line; which is similar to that of boric acid, ZnO has a network-loosening action. In particular on processing of the glasses by the float process, the ZnO content is preferably restricted to rather low amounts (xe2x89xa62% by weight) or ZnO is omitted entirely.
The glasses can contain up to 5% by weight, preferably up to 3% by weight, particularly preferably up to 2% by weight, of ZrO2. ZrO2 increases the heat resistance of the glasses. However, a disadvantage is its sparing solubility, which can result in ZrO2-containing melt relics, so-called zirconium nests. ZrO2 is therefore preferably omitted, impurities of up to about 0.5% by weight, for example as a consequence of corrosion of the refractory material of the troughs, not being interfering.
The glasses can contain up to 5% by weight, preferably up to 3% by weight, of TiO2. TiO2 reduces the solarization tendency of the glasses. At contents  greater than 5% by weight of TiO2, colour casts can occur due to complex formation with Fe3+ ions originating from impurities of the raw materials usually employed. TiO2 is therefore preferably omitted.
A constituent of the glasses which is essential to the invention is MoO3. Even small amounts of this component produce high internal glass quality in these high-melting aluminoborosilicate glasses, i.e. high quality with respect to freedom from or a low content of bubbles and streaks. The glasses therefore contain 0.05-2% by weight, preferably 0.1-1.5% by weight, of MoO3. The effectiveness of this constituent compared with other components, such as SnO2 and/or As2O3, is illustrated in the working examples. It was hitherto known that MoO3 acts as decolorizing agent in extremely small amounts in domestic glasses (see Bulycheva et al.: Glass and Ceramics, Vol. 55, Nos. 1-2, 1998, pp. 42 ff).
Owing to the high effectiveness of MoO3, the addition of further fining agents is unnecessary. Such glasses are free from As2O3, Sb2O3 and SnO2, apart from usual impurities. Impurities of the last-mentioned component can enter the glass, for example during production of the glasses in electrically heated troughs through corrosion of the electrode material. The exclusion of the two first-mentioned components enables processing of the glasses by the float process.
In a different type of processing of the glasses, for example in the drawing processes, the glasses may also contain As2O3 and/or Sb2O3 in order further to improve the already high internal glass quality. Irrespective of the processing method, the addition of halides, sulphates and/or CeO2 is also possible.
Furthermore, the glasses may contain P2O5, Ga2O3, Gd2O3, Fe2O3, La2O3, Nb2O5 and/or Ta2O5. Thus, for example, P2O5 and Ga2O3 are favourable with respect to the crystallization stability of the glasses. The other oxides can have an advantageous effect on the refractive index and the chemical resistance.
The glasses can also contain PbO and/or CdO. On processing by the float method, however, these readily reducible constituents should also be omitted.
The sum of the components CeO2, As2O3, Sb2O3, PbO, CdO, P2O5, Ga2O3, Gd2O3, Fe2O3, La2O3, Nb2O3, Ta2O5, halides and sulphates should not exceed 5% by weight in order to avoid adversely affecting the remainder of the property profile of the glasses.
The high internal glass quality of MoO3-containing glasses can be improved further merely through addition of small amounts of SnO2. The glasses may therefore contain up to 2% by weight of SnO2. The glasses preferably. contain at least 0.1% by weight of SnO2 and at most 0.5% by weight of SnO2. At higher contents, crystallization in the glass or corrosion of Pt employed in the hot-shaping area can occur on contact with Pt.
The glasses are alkali-free. The term xe2x80x9calkali-freexe2x80x9d here is taken to mean that they are essentially alkali-free, i.e. they can contain impurities of less than 1000 ppm.
The action of the MoO3 in the glasses according to the invention goes well beyond that of a fining agent: it improves the acid resistance (cf. Examples A2 and V4) and the solarization stability. It also improves the devitrification stability. With a reduced upper devitrification limit (UDL), an increased difference T4-UDL (cf. Examples A7 and V5) arises, which means an increased processing range. In addition, it increases the corrosion resistance of Mo in contact with the glass or with the melt.